It's true that dynamic content is not supported in emails sent via Sales Insight, sorry you had to find out the hard way.
One option I like to suggest is the creation of a request campaign in Marketo. Create a new camapign in Marketo with the trigger "Campaign is Requested (Marketo Sales Insight)" and the flow will just be to send the email containing the dynamic content.
Now in Salesforce rather than selecting the send Marketo email option your sales team will select Add to Marketo Campaign. This sends the lead/contact into the trigger campaign and as the email is sent from your Marketo instance the dynamic content is applied as appropriate.
One issue is that your sales team won't be able to preview or edit the email before it is sent. And you will also have to name the campaign very clearly so that it's obvious for the sales team what email will be sent.